There may be an objectively correct way to throw globs of paint at the wall if I wish to do it in a way that is consistent with certain desired properties given my state of knowledge. That would not make that correct way of throwing globs of paint "true".
A la Jaynes, there is a correct way to assign degrees of belief based on your state of knowledge if you want your degrees of belief to be consistent with certain constraints, but that doesn't make any particular probability assignment "true". Probability assignments don't have truth value, they assign degrees of belief to propositions that do have truth value. It is a category error, under Jaynes perspective, to assert that a probability assignment is "true", or purple, or hairy, or smelly.
Probability assignments don't have truth value,
Sure they do. If you're a Bayesian, an agent truly asserts that the (or, better, his) probability of a claim is X iff his degree of belief in the claim is X, however you want to cash out "degree of belief". Of course, there are other questions about the "normatively correct" degrees of belief that anyone in the agent's position should possess, and maybe those lack determinate truth-value.
Most people here seem to endorse the following two claims:
1. Probability is "in the mind," i.e., probability claims are true only in relation to some prior distribution and set of information to be conditionalized on;
2. Causality is to be cashed out in terms of probability distributions á la Judea Pearl or something.
However, these two claims feel in tension to me, since they appear to have the consequence that causality is also "in the mind" - whether something caused something else depends on various probability distributions, which in turn depends on how much we know about the situation. Worse, it has the consequence that ideal Bayesian reasoners can never be wrong about causal relations, since they always have perfect knowledge of their own probabilities.
Since I don't understand Pearl's model of causality very well, I may be missing something fundamental, so this is more of a question than an argument.